1. Field of the Invention
The present invention relates to a thin type thermal fuse and a manufacturing method thereof, which is used for manufacturing a thermal fuse for protecting a lithium ion secondary battery from excessive discharge and excessive charge, for example.
2. Description of the Related Art
Recently, a large capacity battery such as a lithium ion secondary battery has been employed as a power source for a portable electric device.
In such a large capacity battery, a quite large current may flow therein at the time of charging and discharging and so abnormal heat may be generated due to the excessive charging or the failure of the main device.
In order to obviate such a problem, it has been investigated to employ a thermal fuse so that the fuse senses this abnormal heating thereby to disconnect the battery from a charging power source or to disconnect the battery from the main device.
Such a thermal fuse for protecting the battery is required to be a thin type. As a thin type thermal fuse, there has been known one which is arranged in the following manner. Tip portions of a pair of belt-shaped lead conductors are fixed on one surface of a resin base film. A low melting-point fusible alloy piece is coupled between the tip portions of the belt-shaped lead conductors. A resin cover film is disposed on the one surface of the resin base film. The space between the films at the peripheries of both the resin films is sealed by an adhesive and also the space between the resin cover film and the belt-shaped lead conductors is sealed by an adhesive.
However, the aforesaid thin type thermal fuse becomes likely inoperative by the following reasons, for example. That is, the ratio (surface area/sectional area) of the belt-shaped lead conductor is quite larger than that of a circular lead conductor. An amount of dissipation heat is too large at the time of connecting the low melting-point fusible alloy piece to the lead conductor by the welding. The defective welding connection is likely occurred. This welded connection is in a state that the alloy piece is connected to the lead conductor in a point-fashion at the one portion of melted metal being spread and remaining portions of the melted metal merely contact to the conductors, and so it is difficult to detect the defective welding even by measuring the resistance value. In an alloy type thermal fuse, the low melting-point fusible alloy piece being fused becomes spherical due to the surface tension and then separated into several pieces. On the other hand, in the aforesaid thin type thermal fuse, since the melted alloy contacts in a circular plate fashion to the inner wall of the thin space, the surface area of the melted alloy on which the surface tension acts is small. Accordingly, the separation function of the alloy piece being fused is essentially degraded as compared with that of the aforesaid spherical shape of the alloy piece being fused of the alloy type thermal fuse.